The following discussion of the background of the invention is merely provided to aid the reader in understanding the invention and is not submitted to describe or constitute prior art to the present invention.
Quantum entanglement is a quantum mechanical phenomenon in which the quantum states of two or more objects have to be described with reference to each other, even though the individual objects may be spatially separated. This is in sharp contrast to classical physics—where particle properties and behaviors depend purely on local conditions. Objects are said to be “entangled” when a plurality (e.g., two or more) objects interact in ways such that the quantum state of each particle cannot be described independently—instead, a quantum state must be given for the system as a whole. Examples of entangled states include position, angular momentum, spin, polarization, energy, and time.
Quantum theory was developed in the early 1900's when classical physics could not explain the behavior of atomic and sub-atomic systems or weak fields. There are many unusual properties which occur at the sub-atomic level, one of which is known as entanglement.
Historically, entanglement was first recognized by Einstein, Podolsky, and Rosen (A. Einstein, B. Podolsky, and N. Rosen, “Can quantum-mechanical description of physical reality be considered complete?”, Phys. Rev. 47 777 (1935)) and Schrodinger (E. Schrodinger, “Discussion of probability relations between separated systems”, Proceedings of the Cambridge Philosophical Society, 31: 555-563 (1935); 32: 446-451 (1936)). Over the years, quantum entanglement has been recognized as a physical resource. Like energy, entanglement can be measured and transformed. The recent development of quantum information theory has shown that entanglement can have important practical applications.
The first known experiment showing polarization measurements on two opposite propagating photons was published by Pryce and Ward (M. Pryce and J. Ward, “Angular correlation effects with annihilation radiation”, Nature 160, 435 (1947)). The early demonstrations of photonic entanglement were centered on annihilation processes, e.g., the decay of gamma particles and the photon emissions which followed. Later, in late 1960s, researchers began to connect the emission of optical entanglement with parametric interactions.
Parametric interactions, which were first studied by Faraday and Lord Rayleigh in the nineteenth century, received renewed attention during this time as a result of their ability to be utilized as microwave amplifiers. Optical parametric interactions within a nonlinear crystal are viewed positively today, in part because they can be utilized for production of entanglement. In contrast, these interactions were viewed as being detrimental to the desired effects of the 1960s. The sentiment changed from “optical parametric noise” to “parametric down-conversion,” largely due to the work of Burnham and Weinberg (D. Burnham and D. Weinberg, “Observation of simultaneity in parametric production of optical photon pairs”, Phys. Rev. Lett. 25, 84 (1970)).
Quantum information science has only recently become a widely recognized field of scientific inquiry. Interest and developments in the field increased greatly in 1994 when Peter Shor discovered a quantum algorithm for factoring large integers in polynomial time (P. Shor, in Proceedings of the 35th Annual Symposium on Foundations of Computer Science, S. Goldwasser, ed., (IEEE Computer Society, Los Alamitos, Calif.), pp. 124-134 (1994)). This discovery sparked a new interest in the abstract notion of quantum computing originally put forth by Paul Benioff, Richard Feynman and David Deutsch in the early 1980s.
The use of quantum effects for communication security were proposed around the same time, in the form of quantum key distribution (QKD) (C. Bennett and G. Brassard, “Quantum cryptography: Public-key distribution and coin tossing,” in Proceedings of IEEE International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, (IEEE Press, 1984), pp. 175-179; C. Bennett and G. Brassard, “Quantum public key distribution,” IBM Technical Disclosure Bulletin 28, 3153-3163 (1985)). Today, commercial prototypes of many quantum technologies are on display. As the technology expands, the demand for more reliable and efficient entanglement sources has and likely will follow.
Quantum entanglement is required for long distance quantum communications and large-scale quantum computing networks. One of the most promising quantum computing architectures, measurement-based quantum computation, is also particularly well-suited for optical implementation. Currently, the best way for generating optical entanglement is via parametric down-conversion, formerly known as parametric noise. The quality of an entangled photon source is commonly characterized by its brightness, that is, the number of generated pairs per mW of pump power and per nm of generated bandwidth, as well as the purity of the entangled state, or visibility. Improvements in source brightness, visibility, and fidelity are constantly being sought.
Early success of parametric down-conversion for entanglement distribution came, primarily, from two major advances in methodology. These techniques, which exploit the geometry of non-collinear parametric down-conversion emissions, were both proposed and realized by Kwiat et al. (P. Kwiat, K. Mattle, H. Weinfurter, and A. Zeilinger, “New High-Intensity Source of Polarization-Entangled Photon Pairs”, Physical Review Letters 75, 4337 (1995); P. Kwiat, E. Waks, A. White, I. Appelbaum, and P. Eberhard, “Ultra-bright source of polarization-entangled photons”, Physical Review A 60, 773 (1999)).
Early demonstrations of polarization entanglement primarily utilized beta-Barium Borate (β-BaB2O4, hereinafter “BBO”) or Potassium Titanium Oxide Phosphate (KTiOPO4, hereinafter “KTP”) crystals that produced spatially-separated entangled beams.
More recently, progress has been made in the collinear regime. The success of collinear parametric down-conversion is due to a crystal manufacturing procedure that yields a periodic nonlinearity to the crystal structure. Emissions within periodically-poled crystals can occur with non-critically phase-matched configurations in materials with large nonlinear coefficients. This has led to significant increases in entangled source brightness (C. Kuklewicz, M. Fiorentino, G. Messin, F. Wong, and J. Shapiro, “High-flux source of polarization-entangled photons from a periodically poled KTiOPO4 parametric down-converter”, Physical Review A 69, 013807 (2004)). Some of the brightest, high-visibility sources of polarization entangled photons demonstrated, to date, utilize periodically-poled crystals in a waveguide structure. Waveguide periodically-poled KTP allows a pair generation rate that is more than 50 times higher (or brighter) than the non-periodically poled, non-waveguide bulk crystal KTP generation rate (M. Fiorentino, S. Spillane, R. Beausoleil, T. Roberts, P. Battle, and M. Munro, “Spontaneous parametric down-conversion in periodically poled KTP waveguides and bulk crystals”, Optics Express 15, 7479 (2007)).
Source performance also becomes better as new engineering techniques and models emerge, e.g. determination of optimal focusing techniques to enable better fiber coupling (R, Bennink, Y. Liu, D. Earl, and W. Grice. “Spatial distinguishability of photons produced by spontaneous parametric down-conversion”, Physical Review A 74, 023802 (2006)). System design can help the performance as well, e.g. improved mounting of a non-linear crystal by encapsulating it within an optically clear material (P. Kwiat, PhD Thesis; “Nonclassical effects from spontaneous parametric down-conversion: adventures in quantum wonderland.”).
Patents related to packaging an entangled photon source include U.S. Pat. No. 6,897,434, “All-fiber photon-pair source for quantum communications,” issued May 24, 2005 to Kumar. Kumar describes a source and/or method of generating quantum-entangled photon pairs using parametric fluorescence in a fiber whose dispersion zero is close to that of the pump wavelength, and specifically, a Sagnac loop at wavelengths around 1550 nm, with detectors in “that window (1000-1600 nm).” A commercial product (EPS-1000) by the company NuCrypt, LLC, claims to practice the teachings of this patent.
Another patent, U.S. Pat. No. 6,424,665 to Kwiat, “Ultra-bright source of polarization-entangled photons,” describes a polarization entangled source using spontaneous parametric down-conversion in a multi-crystal geometry.
Emerging applications for quantum technology create an increasing demand for ever more stable, efficient, high-quality sources of entangled photons. There is therefore a need for a source that can be readily configured and provided to an end-user to produce a rugged, bright, and flexible source to serve the quantum sensing, quantum cryptography, and quantum computing fields.